1. Field of the Invention
The invention relates to a current-perpendicular-to-the-plane (CPP) magnetoresistive (MR) sensor with an antiparallel free (APF) structure.
2. Background of the Invention
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu) or silver (Ag) or alloys thereof. One ferromagnetic layer adjacent the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference layer. The other ferromagnetic layer adjacent the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the reference-layer magnetization due to the presence of an external magnetic field is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as current-perpendicular-to-the-plane (CPP) sensor.
In addition to CPP-GMR read heads, another type of CPP sensor is a magnetic tunnel junction sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a CPP-TMR sensor the current tunneling perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. In a CPP-GMR read head the nonmagnetic spacer layer is formed of an electrically conductive material, typically a metal such as Cu or Ag. In a CPP-TMR read head the nonmagnetic spacer layer is formed of an electrically insulating material, such as TiO2, MgO or Al2O3.
In CPP-MR sensors, it is desirable to operate the sensors at a high bias or sense current density to maximize the signal and signal-to-noise ratio (SNR). However, it is known that CPP-MR sensors are susceptible to current-induced noise and instability. The spin-polarized bias current flows perpendicularly through the ferromagnetic layers and, if it is above a critical current density, produces a spin-torque (ST) effect on the local magnetization. This can produce fluctuations of the magnetization, resulting in substantial low-frequency magnetic noise if the sense current is too large. CPP-MR sensors with an antiparallel free (APF) structure have been shown to have a higher critical current density, so that they are less susceptible to current-induced noise and instability when the current is applied so that electrons flow from the free layer to the reference layer. An APF structure comprises a first free ferromagnetic layer (FL1), a second free ferromagnetic layer (FL2), and an antiferromagnetic coupling (AFC) layer between FL1 and FL2. The AFC layer couples FL1 and FL2 together antiferromagnetically with the result that FL1 and FL2 maintain substantially antiparallel magnetization directions during operation of the sensor.
The sensor stack in a CPP-MR read head has an edge that faces the disk with a width referred to as the track width (TW). A layer of hard magnetic material is typically used to bias or stabilize the magnetization of FL1 and is deposited on both sides of insulating material on each side of the TW edges. As the data density increases in magnetic recording disk drives, there is a requirement for a decrease in the read head dimensions, more particularly the TW. However, the effective or “magnetic” TW does not decrease linearly with a decrease in the physical TW because of side reading of data bits from adjacent tracks. To overcome this problem, side shields of soft magnetically permeable material located on the sides of the sensor TW edges have been proposed to absorb magnetic flux from data bits in adjacent tracks. The side shields replace the hard magnetic biasing material.
A problem with a CPP-MR sensor with an APF structure and soft side shields is that the magnetization of FL2 is inherently unstable. This is because its magnetization direction is antiparallel to the magnetization direction of the soft side shields that provide stabilization for the magnetization direction of FL1. Also, because the field from the magnetization of the soft side shields is more uniform across the entire thickness of the APF structure, the destabilizing effect on FL2 is more pronounced than if the biasing were provided by hard bias layers at the side edges of just FL1.
What is needed is a CPP-MR sensor with an APF structure and soft side shields that has improved magnetic stability of FL2.